The present invention relates to the recognition of a useful signal in a measurement signal.
In general, the measurement of signals can be roughly divided into a) the recognition of individual, more or less singular events and b) the monitoring of more or less frequently recurrent, essentially periodical signals. In both cases, superimposing disturbances limit the confidence level of the measurement and it is desirable to avoid, suppress or filter these disturbances.
Periodical signals are herein understood to mean signals in which the useful signal has at least a periodical component at least in a given time slot, but whose frequency may be time-dependent.
Particularly in the medical field of patient monitoring, the recognition of the useful signal and suppression of disturbances is essential, because disturbances lead to false interpretations of the measured values or may render the measurement as a whole unusable.
A measurement which has proved to be very sensitive to disturbing influences, is the pulsoximetric determination of the oxygen content of blood, because pulsoximetry is often more affected by motion artifacts than by the pulse signal determining the blood oxygen content. Pulsoximetry relates to the non-invasive, continuous determination of the oxygen content of blood (oximetry), based on the analysis of the photospectrometrically measured pulse. To this end, it is necessary that a pulse curve (plethysmogram) is available in the case of a plurality of wavelengths. In practice, substantially all apparatuses operate at two wavelengths only, so that low-cost, compact solutions are possible. The photometry principle is based on the fact that the quantity of the absorbed light is determined by the degree of absorption of a substance and by the wavelength. Pulsoximeters utilize the aspect that the arterial blood volume, and only the arterial blood volume, pulsates in the rhythm of the heartbeat. In order to determine the value of oxygen saturation from the determined measured data, a ratio is derived from the measured data, which ratio then represents the oxygen saturation value. The fundamental aspects and fields of use of pulsoximetry are generally known and frequently described, particularly in EP-A-262778 (with a good theory outline), U.S. Pat. No. 4,167,331, or by Kxc3xa4stle et al. in xe2x80x9cA New Family of Sensors for Pulsoximetryxe2x80x9d, Hewlett-Packard Journal, vol. 48, no. 1, pp. 39 to 53, February 1997.
For the pulsoximetric measurement, particularly methods in the temporal range, adaptive filter spectral analyses and methods in the temporal frequency range have been proposed as methods of recognizing and suppressing artifacts. A detailed description of these methods of suppressing artifacts (which methods are less interesting within the context of this application) is given in the international patent application in the name of the applicant, filed on the same application date (file 20-99-0010).
While the useful signal should remain possibly unaffected in the above-mentioned methods of recognizing and suppressing artifacts, and only the artifacts should be eliminated, the prior art also discloses methods in which, conversely, (only) the useful signal should be filtered from the measurement signal. In addition to the temporal range method (again less interesting within the context of this invention), particularly those methods in which the measurement signals are examined in the frequency range have proved to be advantageous for determining or filtering a periodical useful signal from a more or less disturbed measurement signal. Such methods for use in pulsoximetry are described, inter alia, in U.S. Pat. No. 5,575,284 (Athan), WO-A-96 12435 (Masimo) or EP-A-870466 (Kxc3xa4stle).
According to WO-A-96 12435, a signal is selected as useful signal after transformation of the pulsoximetric measured values in the frequency range by determining the frequency component having the strongest amplitude.
EP-A-870466, by the same inventor and the same applicant, discloses a method of selecting the pulsoximetric signal in accordance with the physiological relevance of the frequency components. After optional suppression of the DC component of the two pulsoximetric raw, or unconditioned, signals (red and infrared), the unconditioned signal values which are present in a continuous time slot are transformed into the frequency range by means of a Fourier transform (here, Fast Fourier Transformxe2x80x94FFT). Ratios of the coefficients of the amplitude spectrum are formed from the transformed unconditioned signals for all frequency peaks. When the infrared spectrum is graphically plotted in the x direction and the red spectrum in the y direction, a representation having needle-like peaks is obtained. These needles correspond to the peaks of the spectra, with very thin needles being obtained for undisturbed signals and the relevant needles of the fundamental and harmonic waves being superimposed. The angle of the needles with respect to the axes corresponds to the saturation value. Since the representation of the spectra is similar to a pincushion in this case, the method described in EP-A870466 is also referred to as xe2x80x9cpincushion algorithmxe2x80x9d.
To identify the needle representing the pulsoximetric signal, a distance spectrum is first determined in the pincushion algorithm from the complex amplitudes of the red and infrared spectra. The distance spectrum describes the distance between every individual point in the needle diagram from the origin. The individual needles are determined from this distance spectrum by considering the maxima and the attendant foot points. Only those needles which fulfill a series of given criteria are maintained for the further considerations. The reduced selection of needles is subjected to a further classification. Needles representing the useful signal should fulfill the criteria that the peaks fit well in a harmonic frequency range, as many harmonic waves as possible are available, the needles are possibly thin and the frequency of the fundamental wave as well as the saturation value, the perfusion and the pulse rate are within physiological ranges. An overall evaluation for each needle is effected by assigning points or K.O. criteria to each of these criteria. The needle that is given the largest number of points, or in other words, best satisfies the criteria, and has been given at least a minimal number of points is used for determining the output value for the pulsoximetric measured value. Optionally, a comparison with previous output values may be used for plausibility control purposes, and in the case of a significant deviation from the previous output values, the newly determined output value is rejected and no new value will be displayed.
The methods of determining the useful signal by transformation into the frequency range have proved to be clearly less sensitive to disturbances than the filtering method in a temporal range. However, also these methods may yield uncertainties in the frequency range, dependent on the disturbing situation, in which, for safety""s sake, no value or only a questionable value can be supplied.
It is therefore an object of the present invention to enhance the recognition of a periodical useful signal in a determined measurement signal. This object is solved by the characteristic features of the independent claims. Advantageous embodiments are defined in the dependent claims.
According to the invention, the recognition of a periodical useful signal in a (disturbed) measurement signal is effected in several process steps.
In a first step, there is a transformation of the measurement signal for a given time slot into the frequency range. The Fast Fourier Transform (FFT) is particularly suitable for this purpose, but other arbitrary transformations may be used alternatively.
Optionally, the measurement signal may be filtered before or after the transformation. Preferably, such a filtering is effected, for example, by reducing the DC component (particularly as described in EP-A-870466 or EP-A-870465) and/or by suppressing transient disturbances, particularly as described in the above-mentioned international patent application in the name of the applicant, filed on the same date of application under file number 20-99-0010.
In a second step, there is an identification of frequency peaks (also referred to as needles) in the transformed time slot of the measurement signal. Such an identification may preferably be effected by using a distance spectrum as described above for the pincushion algorithm in EP-A-870466.
In a third step, there is an assignment of identified frequency peaks of the current time slot to temporal progressions (also referred to as strings) of identified frequency peaks of one or more preceding time slots to the extent that identified frequency peaks are already present. This xe2x80x9cconcatenation of the needles to stringsxe2x80x9d is preferably effected by an initialization, for example, upon a new start, at which the first obtained set of needles is taken to establish a set of strings. A continuous attachment of fitting needles may link up with this string, with a needle being considered to be fitting when there are few deviations as regards predetermined criteria from the last link in the string. The decision whether a needle is to be considered fitting is taken by combining given criteria by means of a fuzzy logic. When no new needle can be assigned to an available string, a gap preferably remains, and the string can be either ended or replaced by a new string if the gap becomes too long (preferably about 30 s).
In a fourth step, there is an assignment of temporal progressions (strings) to one or more families each comprised of a fundamental wave and one or more harmonic waves. Such an assignment or concatenation of the strings to harmonic waves is preferably effected by examining in how far given characteristic features exist between the strings, which features jointly indicate that the strings belong to the same useful signal. Such an examination is preferably performed by combining suitable criteria such as harmonic frequency relations, expected amplitude decrease of the harmonic wave range and/or proportionally equal trend development of the frequencies and/or amplitudes. The combination of the criteria is preferably also effected by means of a fuzzy logic.
In a fifth step, there is a selection of a family as that which should represent the useful signal. The words xe2x80x9cshould representxe2x80x9d are understood to mean that the question whether the selected family also actually represents the useful signal also depends on the selection criteria used. However, since, except in simulated signal ratios, a family can never be recognized with absolute certainty as the xe2x80x9ctruexe2x80x9d family representing the useful signal, the selection should always be considered in the sense of a greatest probability (for the representation of the useful signal by the selected family).
The selection of a family is also preferably effected by combining predetermined criteria such as the existence of a fundamental wave, a first harmonic wave and a second harmonic wave, average fit accuracy of the strings, number of valid needles in a string (i.e. the length of the string), continuity of a string, number of gaps in a string, and the quality of relations between fundamental wave and first harmonic wave, fundamental wave and second harmonic wave, as well as between the first harmonic wave and the second harmonic wave. The combination of criteria is preferably also effected by means of a fuzzy logic.
The selection of a family may be alternatively or additionally effected by means of a plausibility check of the family with respect to previous output values, with the most plausible family being selected.
In a sixth step, a frequency peak of the current time slot is selected from the selected family as that which should represent the measured value of the useful signal in this time slot. The current measured value of the useful signal can then be computed or determined in another way from this selected frequency peak, in so far as this value does not already correspond to the measured value. Also in this case, the words xe2x80x9cshould representxe2x80x9d are understood to mean that the selection is considered in the sense of a greatest probability (for the representation of the measured value of the useful signal by the selected frequency peak).
The selection of the frequency peak representing the current measured value of the useful signal is preferably effected by combining predetermined criteria by means of a fuzzy logic. Criteria relating to a plausibility of the current measured value as compared with previous measured values and/or as compared with expected or useful values are preferably used as criteria.
Optionally, a plausibility check may be performed in the sixth step after selection of the frequency peak, so as to check whether the selected frequency peak actually corresponds to an expected measured value of the useful signal and whether a measured value possibly derived from the selected frequency peak is to lead to an output value, or whether no measured value at all should be outputted for this time slot. Such a plausibility check is preferably performed by comparing the current measured value with previous measured values and/or expected or useful values.
The measured value output for the current time slot is preferably effected together with a quality indicator giving a quantitative statement about the reliability of the supplied measured value. This quality indicator is preferably determined by means of a method as described in the international patent application in the name of the same applicant, filed on the same date of application (file 20-99-0011). The description of the method of determining the quality indicator in this patent application is herein incorporated by reference.
The present invention is preferably used for useful signal filtering of medical measurement signals, for example, in the field of pulsoximetry, blood pressure measurement (invasive or non-invasive) or heart rate determination by means of ECG or ultrasound. The invention is of course not limited to the signal filtering, particularly of the pulsoximetry mentioned above, but can be used for useful signal filtering of arbitrary measurement signals.
The invention may also be used for useful signal filtering in those applications in which the measured values are determined only from one or a plurality of unconditioned signals.
To combine criteria and factors, the principles of the known fuzzy logic are used in preferred embodiments, as are particularly described in Altrock C. xe2x80x9cFuzzy logic: Band 1, Technologiexe2x80x9d, Oldenburg Verlag, Munich, 1995, to which reference is made and will not be further described in this application.
While the history values in the pincushion algorithm described in EP-A-870466 are only considered for selecting a needle to be supplied from the fundamental wave, the method according to the invention takes the history values into account for selecting a harmonic family of strings comprising both a fundamental wave and harmonic wave(s). A family member from the selected family is then selected from the selected harmonic family, which family member should represent the current measured value of the useful signal. The selection of a harmonic family according to the invention thus allows selection of also a harmonic wave as a family member, if the fundamental wave is disturbed, and conversely, and thus clearly enhances the security and reliability of the selection process and thus also the credibility of the possibly determined measured value of the useful signal. A further advantage, particularly also as compared with the mentioned pincushion algorithm is that preceding time slots are also taken into account for the selection of the family members. In the pincushion algorithm, however, preceding time slots are only utilized for a point evaluation for the fundamental wave.
The use of the useful signal filtering according to the invention leads to a noticeable improvement, particularly in the recognition of the useful signal under difficult disturbing conditions. Particularly with a previous disturbance filtering in accordance with the above-mentioned international patent application (file 20-99-0010), increasingly difficult measuring situations can be overcome, and in the further determination and display of a quality indicator in accordance with the above-mentioned international patent application (applicant""s file 20-99-0011), the reliability and confidence can be significantly increased, particularly in difficult measurements as in pulsoximetry.